The invention is in the field of anechoic chambers in general, and particularly relates to testing the efficacity of the radio frequency absorber material that is used to line anechoic chambers.
In the past, there are two methods which have been used to test the RF absorbing material. The most common method is the Naval Research Lab Technique, the NRL Arch Test System, which consists of the use of two relatively high gain antennas (16 dBi minimum standard gain horns) mounted above a metal reference plate. A measurement is made, and then a sample out of absorber material to be measured is then put over the reference plate. In order to measure a -40/50 dB return loss, the sample size must be at least 6 wave lengths on a side. For this reason, when testing with wavelengths below one GHz, this technique becomes difficult to implement.
Below the one GHz a waveguide method has been used with limited success. This technique involves placing the absorbers on a sliding mechanism which moves in and out of the mouth of a large waveguide. Waveguides operating as low as 120 MHz have been constructed.
However, due to the very nature of the construction restraints inherent in the building of the waveguide test fixture, there are many uncertainties built into the system. A major source of error is due to the required transition from a flared rectangular horn section to the square, straight-sided section of the waveguide. If the transition between the flared section and the straight section is not very precisely controlled, large errors are introduced into the measurements. Because the absorber reflectivity can be of the same order of magnitude (45-55 dB) as transition reflections, measurement errors up to five dB are common.
In an attempt to overcome this problem, sliding load techniques have been employed in which the absorber is mounted on a mechanically movable wall which moves through the waveguide. However, in order to measure low operating frequencies the wall must be on the order of 4 feet high and 10 feed wide, so large that difficulties are encountered in moving the wall smoothly through the waveguide. Additionally, operating frequencies are limited to standard guide bandwidths.